1. Field of the Invention
The present invention relates to digital telephony in general and in particular to tone alerting in digital PBX systems, digital key systems, or the like. More particularly still, it relates to digital composite tone alerting in such systems. The apparatus is particularly suitable for systems having digital telephone stations wherein the electro-acoustic transducers are of the piezoelectric type.
2. Related Art
In present telephone systems it is known to provide tone ringing or other audible alerting by using a composite tone comprising two pre-set tone frequencies. In North American telephony, there are five standard tones, two of which provide the composite tone by OR'ing or superposition. The standard five tones are 1000, 667, 500, 333 and 250 Hz. These tones are normally generated locally and audible alerting is restricted accordingly to such fixed tones.